Dr. Robert Taub (l.) testifies in the trial of Sheldon Silver (r.) in U.S. District Court Wednesday, saying he had an arrangement with the former Assembly Speaker that he kept secret. (Jane Rosenberg/for New York Daily News)

The star witness in the government's case against Sheldon Silver said Wednesday the assemblyman asked him to keep their arrangement under wraps.

Columbia University oncologist Dr. Robert Taub said he was introduced to the former Assembly Speaker in the 1980s through a mutual friend, Daniel Chill.

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In 2003 Taub, upon the suggestion of Chill, began directing patients suffering from mesothelioma to Silver, who then referred them to the powerhouse law firm in Weitz & Luxenberg.

"I hoped to develop a relationship with him that would help fund mesothelioma research and help my patients," Taub, 79, said, sporting a colorful bowtie and testifying under a non-prosecution agreement with the feds.

"I think that he conveyed he was pleased with the referrals he was getting."

But under cross-examination Taub — a premier doctor in his field seeking to "incentivize" Silver into funding research — said he thought the arrangement was in his patients' best interests.

"You did not engage in a bribery scheme to exchange patients for grants, did you?" Silver attorney Steven Molo asked.

"I was disappointed," said Taub, who nevertheless continued sending patients to Silver.

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But around 2011, Taub began to send patients to the St. Louis-based Simmons Hanly Conroy law firm, which donated $3.1 million to his research. That prompted Silver, 71, to swing by Taub's office, he testified.

"He said he's not getting as many referrals as before," Taub recalled.

Silver helped Taub's daughter score an unpaid internship with a state judge and also helped Taub's son get a job at a non-profit, he testified. Silver also attended mesothelioma events Taub helped coordinate and made an Assembly resolution in recognition of Taub's work.

Emails introduced as evidence showed Taub thought of Silver as a savvy negotiator "good at getting people to owe him" — but also as a friend.

Their secret arrangement earned Silver $3 million in referral fees over the course of 10 years through at least two dozen patients, prosecutors say.

The alleged quid pro quo with Taub is one of two secret corruption schemes federal prosecutors say Silver, a Lower East Side Democrat, orchestrated.